Integrated optics coupling element comprising a grating produced in a cladding and method for making same

ABSTRACT

The invention relates to an integrated optics coupling element characterised in that it comprises in a substrate ( 11 ) an optical guide core ( 12 ), an optical cladding ( 13 ) independent of the core and surrounding at least one portion of the core in a zone of the substrate called the zone of interaction, in which the cladding has at least in the zone of interaction a modulation of its structure so as to form a grating (R), in which the refractive index of the cladding is different from the refractive index of the substrate and lower than the refractive index of the core at least in the part of the cladding next to the core in the zone of interaction. The invention has applications in particular for the fabrication of gain flatteners for optical amplifiers or even for the fabrication of linear response filters whose wave length is on a spectral band.

TECHNICAL FIELD

The invention relates to an integrated optics coupling elementcomprising an optical grating created in a cladding as well as itsfabrication method.

The invention has applications in all fields requiring a couplingbetween an optical cladding and a guide core or vice versa and inparticular in the field of spectral filtering. It particularly appliesto the creation of gain flatteners for optical amplifiers used forexample in the telecommunications field or even for creating linearresponse filters whose wave length is on a spectral band defined for thespectral recognition in particular for the measurement of spectraloffsets from a power variation for example in the field of sensors.

STATE OF THE PRIOR ART

Currently, the creation of grating coupling elements in optical fibrecladdings is known. In this field, the optical cladding of a fibretraditionally surrounds the fibre core has a refractive index lower thanthat of the core to allow the propagation of a light wave in the core.Conjointly, the optical cladding permits the mechanical support of thecore. The fibre core cannot exist without the cladding.

Furthermore, the grating created in the fibre permits one or more guidedmodes to be coupled in the core of a fibre to one or more fibre claddingmodes or vice versa.

The document (1) whose reference is provided at the end of thedescription illustrates a grating coupling element obtained by etchingthe cladding. However, the creation principle of this type of grating iscomplex, among others it requires the cladding to be etched, which makesthe fibre fragile.

FIG. 1 shows a perspective view of an example of a coupling element inan optical fibre. The fibre 1 comprises a core 3 (shown in dotted lines)and a cladding 5; the latter has been etched for a period A to create agrating R. We can clearly see in this figure that the mechanicalrigidity of the fibre is modified by the etching 7 created in thecladding 5.

In addition to the mechanical difficulties, as the core of a fibrecannot exist without the optical cladding, this dependence restricts thepossibilities of modifying the parameters of the cladding, gratings andsolutions for the design, architecture and integration of the couplingelements in complex systems.

DESCRIPTION OF THE INVENTION

The purpose of this invention is to propose an integrated opticscoupling element comprising an optical grating created in a cladding bymodulation of the cladding structure as well as its creation process.The use of an integrated optics cladding permits the difficulties of theprior art to be overcome by offering in particular more flexibility inthe fabrication of the modulation of the cladding structure and byoffering an element that is not fragile.

One purpose of the invention is also to propose a coupling elementcomprising a grating included in a cladding that is independent of theguide core to which it is associated. By independence of the core andthe cladding, it is meant that can exist in a substrate independentlyfrom one another. In other words, the core can exist without thecladding and the cladding can exist without the core.

More precisely, the integrated optics coupling element of the inventioncomprises in a substrate an optical guide core, an optical claddingindependent of the core and surrounding at least one portion of the corein a zone of the substrate called the zone of interaction, in which thecladding has at least in the zone of interaction a modulation of itsstructure so as to form a coupling grating between the guide core andthe optical cladding, in which the refractive index of the cladding isdifferent from the refractive index of the substrate and lower than therefractive index of the core, at least in the part of the cladding nextto the core in the zone of interaction.

By surrounding, it is meant that the fundamental mode profile of theguide core has a maximum that is included in the index profile of thecladding. In this way, the fundamental mode profile of the core may becompletely or partially included in the cladding index profile, whichresults at structural level in a core positioned anywhere at all in thecladding including at its edge in which case the core may be partiallyoutside of the cladding.

The zone of interaction corresponding to a grating coupling zone in asubstrate will also be called “artificial cladding grating” (ACG). Infact, in this zone, the cladding is artificially created in thesubstrate and independently of the core.

The grating formed from the cladding is capable of coupling the one ormore core modes to one or more cladding modes or vice versa.

In a first advantageous embodiment, the modulation of the claddingstructure is a modulation of its section and preferably of its width,considered in a direction perpendicular to the direction of propagationof the modes.

In a second advantageous embodiment, which may be combined with thefirst mode, the modulation of the cladding structure is a modulation ofthe position of the cladding with respect to the core.

The fabrication of the integrated optics cladding permits it to beobtained by a modification of the refractive index of the substrate, inparticular by implantation or ionic exchange. Consequently, themodulation of the cladding structure can be obtained without etching orfusion as in the prior art.

The solution of the invention therefore offers advantages such as thesimplicity of creation and sturdiness of the coupling element.

Furthermore, the independence between the core and the cladding allows ahigher number of combinations to be created by varying not just the sizeof the cladding but also the position of the core in the cladding. Theindependence of the cladding and the core also permits easy integrationof the coupling element of the invention into a complex architecture.

The grating of the invention may comprise one or more elementarygratings, each elementary grating creating an elementary zone ofinteraction.

The effective index n⁰ _(eff) of the mode spreading in the core dependson the surrounding medium. According to the cladding index and itsextent in the substrate, the value of the effective index of the coremode changes. In this way, by periodically or pseudo-periodicallymodulating the cladding structure, this variation can be transmitted tothe effective index value of the core and thus induce a coupling betweenthe one or more core modes and the one or more cladding modes and inthis way create a grating.

The use of the modulation of the cladding structure is particularlyadvantageous to create a grating. In fact, one of the factorsrestricting the parameter adjustment of the coupling coefficient desiredfor the grating is provided, in the case of masks being used, by thesize of the minimum pattern of the mask lithography permitting thegratings to be created. As this limit is identical for the core and thecladding, it can be easily understood that it is easier to obtain slightvariations on n⁰ _(eff) by varying the structure of the cladding.Consequently, grating type component applications, in particularapodised, are thus favoured.

In a first embodiment, the grating formed by the modulation of thecladding structure is an apodised grating.

In a second embodiment, the grating formed by the modulation of thecladding structure is a chirped grating.

As we have already seen, the cladding structure has an influence on theeffective index of the core mode. Whereas the value of the resonancewave length of the ACG for a coupling from the 0 mode of the core to thej mode of the cladding depends on the effective index values as shown bythe following equation: $\begin{matrix}{\lambda_{0j} = {\Lambda \times \left( {n_{eff}^{0} - n_{eff}^{j}} \right)}} & (1)\end{matrix}$

-   -   Λ is the period of the grating.

A variation in the size of the cladding and/or de its position withrespect to the core therefore permits the value of λ_(0j) to beaccorded.

Coupling the core for example to the cladding (the same logic can beused for coupling the cladding to the core), results in a transfer ofenergy between the guided mode of the core and that of the cladding forwave lengths of λ_(0j). The energy coupled in the cladding modes is thenguided in the cladding generally with losses.

The modification of λ_(0j) therefore passes by adjustment of theparameters of Λ and/or the distribution of the effective indices of thevarious modes.

The efficiency of the coupling between the modes depends on the lengthof the grating and the coupling coefficient K_(0j) between the 0 and jmodes. This coefficient is provided by the spatial recovery integral ofthe 0 and j modes, weighted by the index profile induced by the grating.We therefore have a relationship of the type:K _(OJ)∝∫∫ξ₀·ξ_(j)*·Δn·ds  (2)

-   -   where:        -   ξ₀ and ξ_(j) are the transversal profiles of the 0 and j            modes and ξ_(j)* is the complex conjugate of ξ_(j)        -   Δn is the amplitude of the effective index modulation            induced by the grating in a plane perpendicular to the            direction of propagation of the wave,    -   ds is an integration element in a plane perpendicular to the        axis of propagation of the wave.

The modification of K_(0j) is obtained by varying the profile of themodes and/or the index profile induced by the grating, in other words byvarying the opto-geometrical characteristics of the cladding and/or ofthe core (dimensions, index level, etc.) and/or the characteristics ofthe grating (Δn, position of the grating with respect to the core and tothe cladding, etc.).

As concerns a cladding, the larger its dimensions and index level, themore cladding modes will be allowed to spread and the more spectralfiltering bands will be possible. This may be an advantage if seekingmultiple filtering or to have more flexibility in the selection of afiltering mode.

If seeking to limit the number de cladding modes that can be coupled, itis of interest on the contrary to reduce the opto-geometrical dimensionsof the cladding.

As concerns the core, its dimensions and index level determine thecharacteristics of the mode, which spreads. Furthermore, the more theindex differences between the core, the cladding and the substrate arehigh, the more there will be potentially a chance of having couplingsfor low grating periods as shown by the equation (1) (at a wave lengthof given resonance, the period is inversely related to the indexdifference between the guided mode of the core and the cladding mode).

By modifying the position of the core, the grating and the cladding,different couplings can be generated. In fact, we can clearly see fromthe equation (2) that the coupling force depends on the relativeposition in the plane transversal to the axis of propagation of theprofiles of the cladding mode, of the guided mode of the core and thegrating.

In particular, from the equation (2), it can easily be shown that adecentration δx of the core with respect to the cladding permits K to beincreased.

Also, in one embodiment of the invention the core of the couplingelement is totally or partially decentred with respect to the cladding.

By spectral band it is meant a band with a set of wave lengths whosecentral wave length and band width are determined, given that a lightwave can comprise one or more several spectral bands.

In the invention, the cladding and the core exist independently from oneanother in the substrate, which is not the case in the prior art. Thisindependence permits more flexibility in the creation of the couplingelement. In particular, the core can no longer be situated in thecladding outside of the zone of interaction but solely in the substratewhich permits the optical isolation of the core. In this way thecladding only influences the propagation of a light wave in theassociated guide core in the part surrounding the core and the claddingcan guide or transport light waves independently of the core.

The substrate may of course be made using a single material or by thesuperposition of several layers of materials. In this case, therefractive index of the cladding is different from the refractive indexof the substrate at least in the layers next to the cladding.

Advantageously, each elementary cladding has a refractive index higherthan that of the substrate.

In the invention, the guide may be a planar guide when the light isconfined in a plane containing the direction of propagation of the lightor a micro guide, when the light is confined in two directionstransversal to the direction of propagation of the light.

The grating may be formed by an elementary grating or a set ofelementary gratings in series. The characteristics of the zone ofinteraction of the coupling element are such that they permit thedesired light spectrum to be obtained at the output of this element.

In one preferred embodiment, the cladding and/or the guide core, may becreated by any type of technique which permits the refractive index ofthe substrate to be modified. In particular we can mention the ionexchange techniques, ionic implantation and/or radiation for example bylaser exposure or laser photo inscription or even the depositing oflayers.

The technology of ion exchange in glass is particularly interesting butother substrates apart from glass may of course be used such as forexample crystalline substrates of the KTP, LiNbO₃ or even the LiTaO₃type.

When the cladding is created from a mask, the grating pattern isadvantageously obtained by the same mask.

The invention also relates to a method for fabricating an integratedoptics coupling element as previously defined, the cladding and theguide core being respectively created by modification of the refractiveindex of the substrate so that at least part of the cladding next to thecore and at least in the zone of interaction, the refractive index ofthe cladding is different from the refractive index of the substrate andlower than the refractive index of the core and so that the cladding inthe zone of interaction comprises a modulation of its structure capableof forming the grating.

In one preferred embodiment, the method of the invention comprises thefollowing steps:

-   -   a) introduction of a first ionic species in the substrate to        permit the optical cladding to be obtained after step c),    -   b) introduction of a second ionic species in the substrate to        permit the guide core to be obtained after step c),    -   c) burying of the ions introduced in steps a) and b) to obtain        the cladding and the guide core,

The order of the steps may of course be inverted.

The first and/or the second ionic species is/are advantageouslyintroduced by an ionic exchange, or by ionic implantation.

The first and second ionic species may be the same or different.

The introduction of the first ionic species and/or the second ionicspecies may be made with the application of an electrical field.

In the case of an ionic exchange, the substrate must contain ionicspecies capable of being exchanged.

According to one preferred embodiment, the substrate is made of glassand contains Na⁺ ions introduced beforehand, the first and second ionicspecies are Ag⁺ and/or K⁺ ions.

In a first embodiment, step a) comprises the creation of a first maskcomprising a pattern capable of creating the cladding, the first ionicspecies being introduced through this first mask and step b) comprisesthe elimination of the first mask and the creation of a second maskcomprising a pattern capable of creating the core, the second ionicspecies being introduced through this second mask.

The first mask comprises a pattern whose structure is modulated toobtain the desired structure modulation of the cladding permitting thegrating to be formed.

In one variant of the embodiment, the first mask comprises a uniformpattern, in which the modulation of the cladding structure is obtainedafter elimination of the first mask by localised heating of thecladding, by any known means.

In a second embodiment, step a) comprises the creation of a maskcomprising a pattern capable of creating the cladding and the core, theintroduction of the first and the second ionic species of steps a) andb) being carried out through this mask; the modulation of the claddingstructure is advantageously obtained in this case by localised heating.

The masks used in the invention are for example made of aluminium,chrome, alumina or dielectric material.

In a first embodiment of step c), the first ionic species is buried atleast partially prior to step b) and the second ionic species is buriedat least partially after step b).

According to a second embodiment of step c), the first ionic species andthe second ionic species are buried simultaneously after step b).

According to a third embodiment of step c), the burying comprises thedepositing of at least one layer of refractive material, whose index isadvantageously lower than that of the cladding, on the surface of thesubstrate.

This mode may of course be combined with the two previous modes.

Advantageously, at least part of the burying is carried out with theapplication of an electrical field.

Generally prior to burying with an electrical field and/or thedepositing of a layer, the process of the invention may comprise amongothers burying by re-diffusion in an ionic bath.

This step of re-diffusion may be carried out partially prior to step b)to re-diffuse the ions of the first ionic species and partially afterstep b) to re-diffuse the ions of the first and second ionic species.This re-diffusion step may also be carried out completely after step b)to re-diffuse the ions of the first and second ionic species.

By way of example, this re-diffusion is obtained by plunging thesubstrate in a bath containing the same ionic species as that previouslycontained in the substrate.

Other characteristics and advantages of the invention will becomeclearer from the following description, In reference to the figures ofthe appended drawings. This description is given by way of illustrationand is not restrictive.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 already described, shows diagrammatically an optical fibrecomprising a grating created by etched grooves in the cladding,

FIG. 2 shows diagrammatically in cross section, a first example of anembodiment of a coupling element of the invention,

FIG. 3 shows diagrammatically in cross section, a variant of theembodiment of the coupling element of FIG. 2,

FIG. 4 shows diagrammatically in cross section, a second example of anembodiment of a coupling element of the invention,

FIG. 5 shows diagrammatically in cross section, a third example of anembodiment of a coupling element of the invention,

FIG. 6 shows diagrammatically in cross section, a fourth example of anembodiment of a coupling element of the invention,

FIG. 7 shows diagrammatically in cross section, an example of anapplication of the coupling element of the invention,

FIGS. 8 a to 8 d show diagrammatically an example of an embodiment ofthe invention,

FIGS. 9 a and 9 b show diagrammatically a variant of an embodiment ofthe invention, and

FIGS. 10 a and 10 b show diagrammatically examples of embodiments of themask permitting a cladding with section modulation to be obtained.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIGS. 2 to 7 show examples of embodiments of coupling elements in crosssections containing the direction of propagation x of the light waves inthe core of the said element. In order to simplify things, the directionof propagation is shown contained in a same plane, it of course beingunderstood that the element core may be buried at variable depths.

FIG. 2 shows diagrammatically in cross section, a first example of anembodiment of a coupling element of the invention.

This figure shows a substrate 11 in which a cladding 13 and a core 12are created. The cladding 13 comprises a modulation of its width(considered in a direction y perpendicular to the direction ofpropagation x) in a zone I of the cladding, called the zone ofinteraction. This width modulation creates a grating R with a pitch Λcapable of coupling one or more core propagation modes to one or morecladding propagation modes or vice versa.

The core exists independently of the cladding. It has a constant sectionand in this example traverses the cladding and in particular the zone ofinteraction I.

In this example, the cladding has a section which varies sinusoidallywith the pitch Λ. To simplify this figure, only four grating periodshave been shown.

FIG. 3 shows diagrammatically in cross section, a variant of anembodiment of the coupling element of FIG. 2. This element differs fromthat of FIG. 2 by a core 14 which is decentred with respect to the axisof symmetry of the cladding in the direction x. This variant permits aparameter element to be added concerning the coupling coefficientbetween the cladding and the core by the grating.

FIG. 4 shows diagrammatically in cross section, a second example of anembodiment of a coupling element of the invention.

As in FIG. 2, this coupling element comprises in a substrate 11, a core12 which traverses the cladding 15 in its axis of symmetry considered inthe direction x.

The cladding also has a modulation of section, creating a grating R. Inthis example, the grating is an apodised grating. In fact, thepseudo-sinusoidal pattern of the grating is not constant decreases atboth ends. This is the principle of apodisation that means that thedisruption which generates the coupling phenomenon in the zone ofinteraction I appears and progressively disappears along the propagationof the one or more modes.

The variation of section of the cladding induces disruptions whoseconsequences may be much less important than in the case of thevariation of the core section (in particular due to the dimensions ofthe cladding). The modulation of the section of the cladding of theinvention thus makes apodisation easier.

Other artificial cladding gratings may be created from a variation ofthe cladding structure. By way of example, FIG. 5 shows a chirped typegrating for which the pitch of the section modulation of the cladding 17evolves. The other elements of this figure are the same as those of FIG.4 and have the same references.

It is also possible to combine the different examples of embodiments ofthe coupling element of the invention and create for example a gratingthat is both chirped and apodised.

It is difficult to create apodised or chirped gratings by etching,especially for apodisation which requires good control, distributedalong the length of the grating, from the cladding variation. The use ofgratings created according to the invention is particularlyadvantageous.

FIG. 6 shows diagrammatically in cross section a coupling element of theinvention in which the modulation of the cladding structure is createdby modulation of the position of the cladding with respect to the core.

Consequently, we can see on this figure the substrate 11 in which acladding 18 is created that is traversed by the core 12.

In this example, the section of the cladding is constant but itsposition in the cross section plane of the figure follows with respectto the axis x, a sinusoidal function of period Λ.

Of course, these different variants of the invention may be combinedwith one another.

The fabrication of the grating of the invention by modulation of thecladding structure permits a core with a constant section to be created.This point is of particular interest when the coupling element isintegrated into a more complex architecture. In this case in fact, thecoupling element is associated to the rest of the component by creatingsimply the cladding in a zone of the substrate comprising the core,which permits the operation of the component to be checked without theartificial cladding grating without having to make another mask for thepart of the core that is to be associated to the zone of interaction.

By way of example, FIG. 7 shows a coupling element that is in factintegrated into an optical architecture, in this example thearchitecture is an integrated optics coupler in a substrate 21.

In this way, the coupler comprises in the substrate 21, two guide cores24 and 25 which are close to one another in a coupling zone 26 in orderto permit an energy exchange from one of the guides to the other andvice versa. The core 24 is associated among others after the couplingzone to a coupling element 30 of the invention. This coupling element isformed for example by a cladding 31 comprising a modulation of itssection and by the part of the core 24 which traverses the cladding.

Thus, when a light wave penetrates the core 24 by one end 22, it isfirst of all split in the coupling zone into two parts, one part of thewave continues to be transported by the guide 24 whilst the other partis transported by the core 25. The part of the wave transported by thecore 24 is filtered by the coupling element 30 before leaving by the end28 of the guide. The end 27 of the coupler directly transmits the partof the wave coupled by the coupling zone in the core 25. We consequentlyobtain at the output, a filtered signal and a reference signal.

One application of the component of FIG. 7 may be for example a spectraldetection system. In fact, if the coupling element 30 has a wave lengthlinear response, the end 28 of the core 24 can provide a signal thatdepends on the wave length, whereas the end 27 provides astandardisation signal permitting the spectral characterisation forexample of the position of a fine emission ray in the analysis spectrum.

This coupler may be advantageously optimised, before the couplingelement 30 is created; this is especially advantageous for balancing thetwo output ends 27 and 28.

FIGS. 8 a to 8 d show diagrammatically an example of an embodiment of acoupling element of the invention (for example that of FIG. 2) using ionexchange technology and masks.

These figures are cross sections in a plane perpendicular to the surfaceof the substrate and perpendicular to the direction x of propagation.

FIG. 8 a shows the substrate 11 containing B ions.

A first mask 61 is created for example by photolithography on one of thefaces of the substrate; this mask comprises an opening determinedaccording to the dimensions (width, length) and the pattern of thecladding 13 that we wish to obtain. The mask 61 thus comprises the samemodulations as those that we wish to create in the cladding.

A first ionic exchange is therefore created between the A ions and the Bions contained in the substrate, in a zone of the substrate situatednext to the opening of the mask 61. This exchange is obtained forexample by soaking the substrate equipped with the mask in a bathcontaining A ions and by possibly applying an electrical field betweenthe face of the substrate on which the mask is located and the oppositeface. The zone of the substrate in which this ionic exchange takes placeforms the cladding 13.

To bury this cladding, an A ion re-diffusion step is carried out with orwithout the use of an electrical field applied as previously described.

FIG. 8 b, shows the cladding after it has been partially buried. Themask 61 is generally removed before this step.

The creation of the cladding of the invention is therefore similar tothe creation of a guide core but with different dimensions.

The following step shown in FIG. 8 c consists of forming a new mask 65on the substrate for example by photolithography after possibly cleaningthe face of the substrate on which it is created. This mask comprisespatterns capable of permitting the creation of the core 12.

A second ionic exchange is then created between the B ions of thesubstrate and the C ions which may or may not be the same as the A ions.This ionic exchange may take place as previously described by soakingthe substrate in a bath containing C ions and by possibly applying anelectrical field.

Finally, FIG. 8 d shows the component obtained after burying the core 12obtained by re-diffusion of the C ions and final burying of thecladding, with the use or not of an electrical field. The mask 65 isgenerally removed before this burying step.

The conditions of the first and second ionic exchanges are defined inorder to obtain the differences of refractive indices desired betweenthe substrate, the cladding and the core. The adjustment parameters ofthese differences are in particular the exchange time, the temperatureof the bath, the concentration in ions of the bath and the presence orabsence of an electrical field.

As an example of an embodiment, the substrate 11 is made of glasscontaining Na⁺ ions, the mask 61 is made of aluminium and has an openingof around 30 μm wide and a modulation on the opening of between a fewand several dozen micrometers (the length of the opening depends on thedesired length of the cladding for the application in question).

The first ionic exchange is carried out with a bath comprising Ag⁺ ionsat around 20% concentration, at a temperature of around 330° C. and foran exchange time of around 5 min. The cladding thus formed in the glassis then partially buried. This burying step is carried out byre-diffusion in a sodium bath at a temperature of around 260° C. Thelength of this step depends on the depth of burying desired for thefinal component. In this way, for a surface component a duration ofaround 3 minutes is sufficient whereas for a buried component a durationof around 20 minutes will be selected. In this second case, it is alsonecessary to carry out the burying of the cladding under an electricalfield before the second exchange. In this way, a current of 20 mA isapplied between two sodium baths on either side of the plate at atemperature of 260° C. for 10 minutes.

The mask 65 is also made of aluminium and has a pattern opening ofaround 3 μm wide (the length of the pattern depends on the desiredlength of the core for the application in question).

The second ionic exchange is carried out with a bath also comprising Ag+ions at around 20% concentration, at a temperature of around 330° C. andfor an exchange time of around 5 min. Then the core thus formed ispartially buried in the glass by re-diffusion in a sodium bath at atemperature of around 260° C. for 3 min. For a buried component, thisstep is not necessary.

The final burying of the cladding and the core takes place under anelectrical field, with the two opposite faces of the substrate incontact with two baths (in this example sodium) capable of permitting adifference in potential to be applied between these two baths. For asurface component, a duration of less than one minute is sufficient, inthe case of a buried component a duration of around 30 minutes is used,with the burying being carried out with a current of 20 mA at 240° C.

Many variants of the previously described process may be created. Inparticular, the burying steps of the cladding and the core may becarried out as previously described during 2 successive steps but theymay also be carried out simultaneously in certain cases, with the corehaving an ionic concentration higher than that of the cladding, it isburied quicker than the cladding, which permits among others to centrethe core in the cladding.

The difference in concentration between the core and the cladding isgenerally obtained either by re-diffusion in a bath of ions forming thecladding or by a difference in concentration of the ions introduced insteps a) and b).

As we have already seen, to bury the cladding and the core, a variant ofthe process consists of depositing on the substrate 11, a layer ofmaterial 68, shown in dotted lines in FIG. 8 d. In order to make opticalguiding possible, this material must advantageously have a refractiveindex lower than that of the cladding.

Moreover, in this example the cladding is created before the core but itis of course possible to create the core before the cladding.

The fabrication of the component of the invention is not restricted tothe technique of ion exchange. The component of the invention may alsobe created by any techniques which permit the refractive index of thesubstrate to be modified.

Furthermore, as we have already seen, the period, size and position ofthe grating created, with respect to the core and the cladding, areparameters which can be adapted to suit the applications.

FIGS. 9 a and 9 b show in a perspective view a variant of an embodimentof a coupling element of the invention that does not use masks.

Thus FIG. 9 a shows the substrate 11 in which a cladding 60 with auniform structure has been created beforehand, for example by maskingand ion exchange. Localised heating 63 of the cladding is then createdby means of a laser beam 71 (for example a CO₂ type laser) aimed at thesubstrate. This beam is moved along the cladding, by intervalscorresponding to the desired period of the grating. The localisedheating produces re-diffusion of the ions in the cladding, which resultsin both a change in section and index. The grating R is thus created inthe cladding.

After this step (FIG. 9 b), a guide 75 is fabricated in the cladding forexample also by masking and ion exchange in order to obtain the couplingelement of the invention.

In this example of the embodiment, the modulations of the claddingstructure are obtained without modulating the cladding mask pattern. Itis therefore possible to modify the opto-geometrical distribution of thecladding by simply creating periodic or pseudo-periodic localisedheating. This heating can be obtained by all means permitting part ofthe substrate to be heated locally on a zone around the size of thedesired grating period, following the direction of propagation of themodes. The se means may be for example laser exposure or the use of anelectrical arc.

The exposure of the cladding to a laser beam may also be made after theguide core has been created.

FIGS. 10 a and 10 b show diagrammatically examples of embodiments of themasks M1 and M2 permitting a cladding with section modulation to beobtained.

These figures are elevation views of the masks and only show the part ofthe masks permitting the grating to be obtained. The white zones of themask patterns correspond to the openings of the masks.

These masks permit a periodic grating of period Λ to be obtained byvariation of the width of the patterns.

REFERENCE

-   [1]: C. Y. Lin and L. A. Wang, “Loss-tunable long period fibre    grating made from etched corrugation structure”, Electron. Lett., 35    (21), (1999), pp 1872-1873

1. A coupling element, comprising: a substrate; an optical guide coreformed in said substrate; and an optical cladding formed in saidsubstrate, said optical cladding being independent of the optical guidecore and surrounding at least one portion of the optical core in a zoneof interaction, wherein a structure defining the cladding is modulatedat least in the zone of interaction so as to form a coupling gratingbetween the optical guide core and the optical cladding, and wherein arefractive index of the cladding is different from a refractive index ofthe substrate and lower than a refractive index of the core at least ina part of the cladding adjacent the optical guide core in the zone ofinteraction.
 2. The coupling element of claim 1, wherein a section ofsaid structure is modulated.
 3. The coupling element of claim 1, whereina position of said structure with respect to the core is modulated. 4.The coupling element of claim 1, wherein said structure is modulated byionic implantation, ionic exchange or local heating.
 5. The couplingelement of claim 1, wherein said coupling grating formed by modulationof said structure is an apodized grating.
 6. The coupling element ofclaim 2, wherein the coupling grating formed by modulation of thesection is a chirped grating.
 7. A method for fabricating couplingelement, said coupling element comprising a substrate, an optical guidecore, and an optical cladding formed in said substrate, said opticalcladding being independent of the optical guide core and surrounding atleast one portion of the optical core in a zone of interaction, themethod comprising: modifying a refractive index of a substrate to formthe optical guide core; and modifying the refractive index at least in apart of the substrate adjacent the optical guide core and at least inthe zone of interaction to form the optical cladding, wherein arefractive index of the optical cladding is different from a refractiveindex of the substrate and lower than a refractive index of the opticalguide core, and wherein a structure defining the optical cladding in thezone of interaction is modulated to form a grating.
 8. The method ofclaim 7, wherein the refractive index of the substrate is modulated byradiation and/or by introduction of ionic species.
 9. The method ofclaim 8, wherein the substrate includes glass, KTP, LiNbO₃ or LiTaO₃.10. The method of claim 8, further comprising: a) exposing the substrateto a first ionic species, b) exposing the substrate to a second ionicspecies, and c) burying said first and said second ionic species toobtain the optical cladding and the optical guide core.
 11. The methodof claim 10, further comprising: defining a first mask comprising apattern configured to define the cladding, said first ionic speciesbeing introduced through said first mask, removing said first mask, anddefining a second mask comprising a pattern configured to define theoptical core, said second ionic species being introduced through saidsecond mask.
 12. The method of claim 11, wherein the pattern of thefirst mask is configured to define a modulation of said structure toform the grating.
 13. The method of claim 11, wherein the pattern of thefirst mask is uniform, and wherein said structure is modulated by localheating of the optical cladding.
 14. The method of claim 10, furthercomprising: defining a mask comprising a pattern configured to definethe optical cladding and the optical guide core, the first and thesecond ionic species being introduced through said mask, and locallyheating said structure to modulate said structure.
 15. The method for ofclaim 11, wherein said first and second mask are made of chrome, aluminaor dielectric material.
 16. The method of claim 10, wherein said buryingcomprises depositing at least one layer of material with a refractiveindex lower than that of the cladding on the surface of the substrate.17. The method of claim 10, wherein the burying comprises applying anelectrical field to the substrate.
 18. The method of claim 10, whereinthe substrate includes glass and Na⁺ ions, and wherein the first andsecond ionic species include Ag⁺ and/or K⁺ ions.